Q1: Can I increase the output volume at least 6db up without additional amp?

A1: The module has a 1W amp and you don't need to use any additional amp for 6db only. I guess you connected the wrong pin outs from the module. You need to connect the pin 13 and pin 14 of the module to the 1W 8Ohm speaker. Which pins were you using?, pin 8 and pin 9? Through these two pins the module actually need to work with an external amplifier/active speaker.

Q2: Can the module automatically keep max output volume or it will go to default level after every disconnecting/connecting the power voltage?